The present invention relates generally to mobile communications and, more particular, to mobile communications systems utilizing high-speed downlink channels.
Traditional mobile telephony services are essentially symmetric in their use of available bandwidth since both the uplink (i.e., transmissions from the user to the base station) and the downlink (i.e., transmissions from the base station to the user) bandwidth requirements are similar. However, as data-oriented mobile services become more prevalent, many such services require significantly more bandwidth for the downlink than the uplink. In order to address this increased downlink requirement for bandwidth, various schemes have been developed. One such scheme, known as High Speed Downlink Packet Access (HSDPA) is an improved downlink packet data scheme for 3rd Generation (3G) Partnership Project Organization (3GPP) mobile communication systems, such as the Universal Mobile Terrestrial System (UMTS) system that uses a wideband code division multiple access (WCDMA) air interface. As one skilled in the art will recognize, 3GPP is a standards body for a large number of 3G specifications for current and future mobile communication systems, such as the UMTS and WCDMA standards. As one skilled in the art will further recognize, HSDPA is an upgrade to existing 3G systems that improves capacity and leads to higher data transfer rates as compared to 3G systems that have not been upgraded.
HSDPA facilitates such transfer rate improvements by providing new shared and dedicated channels for both the downlink and the uplink. FIG. 1 shows these new shared channels 104 and dedicated channels 103 between, for example, a wireless base station 102 (herein referred to as a Node B) and a user device 101 (herein referred to as user equipment (UE)). In particular, a wideband downlink shared channel, referred to in the art and herein as the High Speed Physical Downlink Shared Channel (HS-PDSCH) 108, is a shared channel among all users in a cell and is optimized for very high-speed data transfer. The HS-PDSCH 108 can provide a maximum theoretical downlink speed of up to 14.4 Mb/s, while prior systems were typically limited to a maximum of less than 2 Mb/s. In addition to the HS-PDSCH channel, HSDPA also utilizes a new control channel, referred to herein as the High Speed Shared Control Channel (HS-SCCH) 107. This control channel functions to transmit various control parameters, for example parameters received from RNC 109, from the Node B to the UE. In the uplink direction, HSDPA uses a low bandwidth uplink channel, referred to herein as the High Speed Dedicated Physical Control Channel (HS-DPCCH) 105, for sending data packet acknowledgements/negative-acknowledgements (ACK/NACK) and channel quality information (CQI) from the UE to the Node B. One skilled in the art will recognize that these HSDPA channels can be used to upgrade prior 3G systems and, therefore, other uplink and downlink channels may also provide lower data rate communication, such as the Uplink DCH and Downlink DCH 106 channels shown in FIG. 1. One skilled in the art will also recognize that such HSDPA cells are typically deployed so that, for one specific UE, only one of several cells in a given area is an HSDPA serving cell (i.e., capable of providing HSDPA service). Thus, in typical arrangements, for one specific UE an HSDPA cell will be surrounded by non-HSDPA radio links to other cells.
While HSDPA is, therefore, extremely advantageous, one limitation in prior HSDPA implementations is that the uplink signal from a particular UE must be synchronized with the high speed downlink channel in order for downlink transmissions to be schedule to that UE. However, in some cases interference, for example originating from other users signals from the same or neighboring cells, prevents the uplink CQI and ACK/NACK information from being received from the UE in the HSDPA serving cell and, therefore, prevents downlink transmissions from being scheduled to that UE. As one skilled in the art will recognize, when this information is not received from the UE, synchronization between the UL and DL channels is lost and, as a result, no transmissions will be transmitted from the HSDPA Node B to the UE. This can result in loss of service, loss of the radio link between the UE and Node B or, in some cases, dropped calls. To date, the most popular prior method of reducing this type of interference is to reduce the downlink CPICH power in particular cells in order to balance the uplink and downlink coverage.